What causes the amniotic sac to break?

Just out of curiosity, what actually causes your amniotic sac to break? Does it just begin to disintegrate after time and can't hold up anymore? Does a contraction finally squeeze it enough to rupture?

Pressure plus a large enough opening for it to bulge out (ie dilation)

Most of the time, a healthy woman's sac will burst spontaneously during hard labor, often during pushing. The pressure of the contractions pops it basically like squeezing a water balloon. Occasionally it does burst before labor starts sometimes from the pressure of the baby's head on a softened cervix.

My mw also mentioned that in her experience, smokers have a weaker sac and are more likely to break early.

But it's totally normal to remain intact until pushing. In fact, in many ways it makes labor easier on both mom and baby. The contractions don't exert as much pressure on the baby's body when cushioned with the fluid and they tend to be a tiny bit gentler for mom, too.

Thank you, mayctober! I was just curious and nothing I've read really answered that question. It's just an interesting concept - I've seen that infections can cause the sac to break prematurely and wondered how that worked. My only experience was the doctor breaking it for me. I figured it was a little more complex than the baby's fingernail poking a hole in the sac!